This invention is generally directed to a threaded male fastener member having a novel point configuration which prevents the fastener member from cross-threading upon engagement with a correspondingly threaded female member and the method of assembly thereof. Either the fastener member or the female member may be driven, the other member being stationary during the assembly. While the discussion that follows is directed primarily to the situation wherein the fastener member is the driven element and the female or internally threaded member is stationary, it should be kept in mind that the fastener member may be stationary and the internally threaded member, such as a nut, may be driven.
During automated or manual assembly of an externally threaded fastener member with an internally threaded member, such as a nut, a tapped bore in a workpiece or an internally threaded rivet staked to a workpiece, one of a number of conditions will occur at assembly. First, proper engagement of the male threaded fastener member with the female threaded member will be obtained which is an acceptable condition. Second, cross-threading will occur which is an unacceptable condition. Third, a no-start will result wherein the male threaded component rotates without effecting threaded engagement with the threaded female member which is also an unacceptable condition.
If proper engagement is obtained, the external thread on the fastener member engages with the internal thread in the female member and the external thread then follows the helix path of the internal thread. The mating of the helix angle of the internal and external threads provides an inclined plane which converts input torque into clamp force in the joint.
Cross-threading occurs when a fastener member is engaged with a female member and the threads are not properly aligned when the threads on the fastener member and in the female member first engage. If the external threads on the fastener member and the internal threads in the female member are not properly aligned at the start of the threading operation, the external thread tends to cross over the crest of the internal thread which can produce deformation of the threads and binding or seizing up of the components before the desired depth of engaging is obtained. Unless the fastener member or the female member are manually realigned at this point, the external thread will wedge against the internal thread thus preventing assembly and/or damaging the threads. Cross-threading is a common occurrence in prior art fastener members, and extremely troublesome in automated assembly operations.
In an automated assembly situation, the occurrence of cross-threading can severely affect productivity. In some cases, assembly workers will have to manually engage the fastener member and the female member for a couple of threads, then apply the automated equipment to these prealigned fastener members.
A no-start condition can occur when a rotating fastener member is misaligned with the tapped bore such that the external lead threads of the fastener member do not engage into the tapped bore in a specified period of time. Thus, the fastener member will "spin" at the top of the tapped bore. In some assembly operations, the end of the fastener member "wobbles" which can cause the fastener member not to engage and which can cause the fastener member to quickly move away from the tapped bore. This quick movement can cause damage to the surface of the joint. For example, if the joint is a painted surface, the fastener member could scratch or otherwise mar the surface.
Prior art fastener members which have attempted to prevent cross-threading within a tapped bore are provided with various designs. These prior art designs generally involve an asymmetrical point or entry end on the fastener member, with only a portion thereof on the asymmetrical end point or entry end. Examples of prior art fastener members include a "P" Point, a MAT Point, a CTP Point which is disclosed in U.S. Pat. Nos. 4,981,406 and 5,419,667, an ACT Point which is disclosed in U.S. Pat. Nos. 4,789,288 and 4,915,560, and a TRU-START which is disclosed in U.S. Pat. No. 5,064,327.
The CTP point, the ACT point, and the TRU-START all employ tapered lead threads inserted within a tapped bore before engagement of the fastener member with the tapped bore. This can result in a wedging action which causes cross-threading. The other point designs have a blunt end which can cause a no-start condition at off-angle conditions.
It is desirable to eliminate the cross-threading and no-start problems. The fastener member of the present invention provides such a fastener member. The present fastener member minimizes the problems found in the prior art by several novel improvements to prior art fastener members. Further, the design of the present invention provides a novel fastener design which is capable of improved performance when used with automated assembly equipment and operations. First, the fastener member resists cross-threading. Second, the fastener member is capable of rapid engagement with the tapped bore to reduce the assembly installation time and to improve the ergonomics of a typical installation. Third, the fastener member has a large "window of engagement" into the tapped bore by compensating for a condition where the fastener member and the tapped bore are out-of-alignment or off-angle from each other by providing a self-aligning point and by compensating for a condition where the fastener member and the tapped bore are offset from each other. Further, the externally threaded fastener member has a radiused portion on the entry end thereof which is relatively smooth and free of sharp edges or corners which could catch or hand-up on the internal threads. These features all work together to provide for a smooth engagement of the fastener member with the tapped bore. The point used in the fastener member of the present invention compensates for wobble at the end of the fastener member which reduces the potential damage to the surface of a joint. Fourth, the point length of the fastener member is minimized which lowers material cost, reduces the weight of the fastener member and minimizes interference problems with the point of the fastener member with other components. Fifth, the fastener member of the present invention lowers manufacturing cost as the fastener member threads and the point are roll threaded in one pass. The structural features of the fastener member of the present invention and manner whereby these features function to attain the improved performance discussed above will become apparent from the discussion as follows, especially with respect to the drawings.